During the production of alkanesulfonic or arenesulfonic acids by methods known to those skilled in the art, the product acids are generally colored, ranging from light yellow to essentially black. Heretofore, various method have been employed to either prevent the formation of this color during the manufacture of the sulfonic acids or to remove the color from the product acids once they have been formed.
For example, U.S. Pat. No. 2,502,619 describes the purification of salts of alkanesulfonic acids to remove colloidal materials and coloring matter by heating the solid alkanesulfonic acid salts at temperatures between about 100.degree. and 250.degree. C., dissolving the thermally treated salt in water, and treating the resulting aqueous solution of the alkanesulfonic acid salt with a granular adsorbent, such as activated carbon. The present inventor has found that the coloring matter in anhydrous alkanesulfonic acid is not substantially removed by treatment with adsorbants, such as activated carbon, and that the dilution of the anhydrous sulfonic acid with water, in order to effect removal of the coloring matter by this method, is not desirable.
U.S. Pat. No. 3,269,927, discloses that color bodies and color-forming materials can be effectively removed from alkanesulfonic acids by subjecting the color-contaminated sulfonic acid to a direct current electrical field. However, this method employs undesirably high current densities (e.g., 10-120 amps). The present inventors have found that using this method results in the formation of sulfuric acid, which is an undesirable impurity in the resultant anhydrous alkanesulfonic acids.
According to U.S. Pat. Nos. 3,413,337 and 3,479,398, alkanesulfonic acids of improved color can be produced by the reaction of saturated hydrocarbons with sulfur dioxide and a catalyst, either by using ozone as the reaction initiator, or by further treating the product alkanesulfonic acids from the sulfoxidation reaction with ozone after removal of the acids from the sulfoxidation reactor. However, the use of ozone for the decolorization of the sulfonic acids requires the use of very high voltages and specialized equipment for the generation of the ozone. This may result in the formation of undesirable impurities, such as sulfuric acid, in the decolorized sulfonic acids.
U.S. Pat. No. 3,666,797, describes a method for the production of light-colored paraffin sulfonic acids through the sulfoxidation of paraffinic hydrocarbons. In this method, the product sulfonic acids are separated from by-product sulfuric acid and concentrated by evaporation of water in the presence of hydrogen peroxide, in order to prevent the formation of extraneous color bodies. However, the use of hydrogen peroxide to decolorize the anhydrous sulfonic acids is undesirable, as this method introduces water into the anhydrous acids and can result in the formation of sulfuric acid.
U.S. Pat. No. 4,197,255 describes the bleaching of C.sub.8-22 olefinic sulfonic acids in an alkaline solution at a pH of approximately 10-14 and a temperature of 30.degree.-80.degree. C., using hydrogen peroxide, chlorine, alkaline metal chlorites, or alkaline metal hypochlorites as bleaching agents. However, this method employs an alkaline medium which is not applicable to the decolorization of anhydrous sulfonic acids. Also, the use of chlorine oxides would introduce undesirable impurities into the anhydrous sulfonic acids.
In Japanese Patent Application (OPI) No. 73/22423 (Chemical Abstracts, 79, 18083 (1973)) it is reported that substantially anhydrous methanesulfonic acid can be decolorized by the passage of chlorine through the acid for two (2) hours at a 135.degree. C. However, this method also introduces undesirable impurities into the sulfonic acid, such as chlorine and/or chloride ions. Also, other impurities, such as sulfuric acid and chlorinated methane sulfonic acid, may be formed during the decolorization treatment.
Therefore, in view of the deficiencies of the known methods as described above, it would be desirable to have a method for decolorizing alkanesulfonic or arenesulfonic acids which does not involve the addition of water or other undesirable impurities to the purified anhydrous sulfonic acid, and which does not result in the formation of undesirable impurities during the decolorization of the sulfonic acid.